This invention relates to formation of a multicolor interference coating for a transparent or opaque substrate. The coating material is an acrylate polymer and different colors are obtained by having different thicknesses of transparent coating in adjacent areas.
Interest has developed in recent years in the protection of currency and other documents from counterfeiting by use of interference filters. The color variations available from interference filters cannot be duplicated by copying machines. The specialized equipment needed for producing the interference filters is not readily available to counterfeiters. Thus, it has been proposed to mark Canadian currency with multicolored interference filter patterns to inhibit counterfeiting. See, for example, "Optical Interference Coatings for Inhibiting Counterfeiting" by J. A. Dobrowolski, et al., Optica Acta, 1973, 20, No. 12, pp. 925-937 and U.S. Pat. No. 5,009,486 by Dobrowolski, et al.
Interference filters have been known for decades, as seen, for example, in U.S. Pat. No. 2,590,906 by Tripp. A typical interference filter has a largely reflective metal film on a smooth substrate. The reflective film is overlain by a thin layer of transparent dielectric material. The filter is completed by a semi-reflective metal layer over the dielectric. A transparent protective coating may be applied over the reflective coating, but does not form part of the interference filter itself.
When an incident light beam encounters the front semi-reflective coating of the interference filter, one fraction of the light is reflected and the other fraction passes through the semi-reflective layer into the dielectric. The transmitted portion of the beam is then reflected by the back reflective layer and retransmitted through the dielectric. A fraction of the reflected wave passes through the semi-reflective front layer where it may constructively or destructively interfere with the reflected light.
The thickness of the dielectric material is a small multiple of a quarter wavelength of light for constructive interference (allowing for the index of refraction of the dielectric material). Thus, when light is reflected from the interference filter, light with the appropriate wavelength has the reflected and transmitted beams in phase for constructive interference. Light of other colors has at least partial destructive interference. Thus, when a reflective interference filter is observed in white light, it reflects a strong characteristic color.
The interference filter has a desirable characteristic as an anti-counterfeiting measure. The color reflected from the filter depends on the path length of light passing through the dielectric material. When the filter is observed with light at normal incidence, a certain color, for example blue, is seen. When the angle of incidence and reflection from the interference filter is more acute, the total path length through the dielectric material is longer than for normal incidence. Thus, when the interference filter is observed at an angle nearer grazing incidence, a longer wavelength color, for example purple, is observed. Such a characteristic change of color, depending on the angle of viewing the interference filter, cannot be reproduced by copying machines.
A similar effect for transmission of a light can be obtained when the interference filter has a thin dielectric sandwiched between two partially reflective layers. One type of interference filter is sometimes referred to as a quarter-wave plate because of its characteristic thickness of 1/4+L wavelength.
To make it even more difficult for counterfeiters, it has been proposed to use interference filter layers having different thicknesses in different areas. Since the color of light reflected from an interference filter is a function of the thickness of the dielectric material, one can thereby achieve a multicolor effect by having different areas of the filter with different thicknesses.
The Dobrowolski concept is to produce an interference filter using an inorganic optical coating material, such as those listed in U.S. Pat. No. 5,009,486. A layer of such material is deposited with a certain thickness. A mask is superimposed and a second layer of that material is deposited over a portion of the first layer. Collectively, these two layers define areas of differing thicknesses and hence, different interference colors.
Such a technique is costly. The metal and dielectric layers are typically deposited on a thin film polyester substrate by a sputtering technique at a rate of about 3 to 10 meters per minute movement of the film past the deposition stations. Much faster deposition is desirable. Furthermore, two separate deposition steps with intervening masking of the surface must be performed to provide the two layers of dielectric which collectively provide a color difference.
It would be desirable to enhance the rate of formation of an interference filter by at least an order of magnitude as compared with the inorganic dielectric materials previously used. It is also highly desirable to provide varying thickness of the dielectric material in the interference filter in a single deposition step for forming a monolithic layer of differing thickness. It is also desirable to deposit the interference filter material in predetermined patterns of differing color.